This invention relates to decaffeination of green coffee beans. More particularly, this invention relates to treatment of green coffee beans prior to or simultaneously with decaffeination. Still more particularly, this invention relates to treatment of green coffee beans by contacting them with an aqueous organic acid solution in connection with supercritical extraction of caffeine.
Many processes are known for decaffeination of coffee. One particularly preferred process is supercritical extraction of caffeine, typically performed using supercritical carbon dioxide, perhaps with cosolvents such as water or so-called enhancers. Such processes are disclosed, for instance, in U.S. Pat. Nos. 4,820,537 and 4,260,639. Such processes practiced to the present time have suffered in their ability to produce coffee comparable in flavor and aroma to non-decaffeinated coffee.
U.S. Pat. No. 4,911,941 to Katz is directed to a method of continuously decaffeinating moistened green coffee beans with supercritical carbon dioxide. The moistened coffee beans are moved through an extraction vessel and are contacted with continuously flowing supercritical carbon dioxide which extracts caffeine from the green coffee beans. The caffeine-laden supercritical carbon dioxide removed from the caffeine extraction vessel is continuously fed to a countercurrent liquid absorber to remove caffeine, but not the non-caffeine solids. The countercurrent absorber exhibits excellent selectivity for caffeine when contacting supercritical fluids containing caffeine and non-caffeine solids. The essentially caffeine free supercritical fluid contains very nearly the same level of non-caffeine solids as it did upon entering the absorber. The Katz patent teaches that if this supercritical fluid (containing non-caffeine solids at substantially the same level as the supercritical fluid contained upon leaving the decaffeinating extraction chamber) is used as the extraction fluid, the supercritical fluid extracts no measurable amount of non-caffeine solids from the green beans as they are decaffeinated.
The wash water from the absorber can be treated by reverse osmosis to recover a caffeine retentate and a permeate stream containing coffee acids which are soluble in supercritical carbon dioxide and other non-caffeine solids. The permeate can be returned to either the extraction vessel or to the absorbing vessel. At best, the Katz patent teaches that a supercritical carbon dioxide which contains non-caffeine solids at a level that is extracted during the normal extraction process is utilized to extract caffeine from moist green coffee beans, that no further extraction of non-caffeine solids would occur.
While supercritical carbon dioxide is an effective decaffeination medium, its solvent action is not specific to caffeine or caffeine precursors. During supercritical decaffeination of green coffee beans, numerous other components are removed including but not limited to waxes, oils and color bodies. It is known that the pH of a coffee beverage contributes to perceived flavor, and that the range of pH for acceptable palatability is from about 4.95 to about 5.15. Others have attempted to control the pH of the final coffee product, i.e., instant or roast and ground coffee, by adding acidic or alkaline substances to the final coffee product to account for unacceptable pH caused by such factors as the botany of the coffee beans, the altitude at which they are grown, the processing method applied to the coffee fruit, the age of the beans when processed and the degree of roasting. See, e.g., Stivetz, How Acidity Affects Coffee Flavor, Food Technology, Vol. 25, No. 5, 1972, pp. 70-77. However, there appears to be no recognition in the art that supercritical decaffeination adversely affects acidity, nor does there appear to be any method disclosed for controlling the acidity of coffee products other than final coffee products.